The invention relates generally to chemical sensors using optical waveguides and more particularly to refractive index type chemical sensors using optical waveguides.
Optical waveguides, structures which guide light by total internal reflection, having been used in chemical sensors. Most waveguide chemical sensors are fiber optic chemical sensors (FOCS) based on optical fibers which guide light through the core by total internal reflection at the core/clad interfacing using a clad of lower refractive index than the core.
U.S. Pat. No. 4,929,049 describes a fiber optical refractive index sensor using a thin film metal clad which allow modulation of transmitted signal by changes in the refractive index of the surrounding medium. C.I.P. application Ser. No. 07/501,144, now U.S. Pat. No. 5,026,139, expands the concept to include selection of thin film metal clads which are specific to analytes of interest and to use combinations of different clad materials of different specificities to determine particular analytes.
U.S. Pat. No. 4,913,519 to Klainer, etal, describes an optical sensor for detection of ice formation or other chemical species in which a striped clad is formed on a fiber optic core or other waveguide structure. The speed stripes of clad material have a refractive index less than the light guiding means and substantially equal to the refractive index of the chemical species to be detected. When the species in not present, the gaps between the clad stripes allow light leakage so that little or no light is transmitted through the waveguide. When the species is present, it fills the gaps to form a complete clad which totally internally reflects light within the waveguide so that light is transmitted therethrough.
P.C.T. patent application PCT/CH86/00072 to Tiefenthaler, et al describes an optical waveguide sensor to selectively determine the presence of substances and the variations of refractive index in measured substances. The sensor is formed of a planar waveguide film, or a narrow strip, with a diffraction grating formed thereon. The thin film waveguide is formed of an oxide or polymer. The grating provides input or output coupling or acts as a Bragg reflector within the waveguide. The sample interacts with the waveguides in the region around the grating. In order to selectively establish the presence of specific substances, a layer of selective chemisorption material is formed on the thin film waveguides. The remainder of the waveguide surface may be covered by a low-index protective layer. According to Tiefenthaler the presence of a chemisorbed layer and/or the modification of the refractive index of the measurement substance modifies the refractive index within the waveguide film, which produces measurable changes in the propagation of light waves therein.
The metal clad FOCS of application Ser. No. 07/501,144, now U.S. Pat. No. 5,026,139 are advantageous for detecting a wide variety of chemical species; however, a disadvantage of the FOCS configuration is the small surface area available and the difficulty in attaching some clad materials to the fiber optic surface. It would be highly advantageous to apply the principles of the metal clad FOCS sensors to a planar waveguide of the type shown by Tiefenthaler. The larger surface area of the planar waveguide makes it much easier to apply the desired coatings, particularly where clad materials of specific crystallographic orientation are utilized. The improved coatings provide a sensor which can operate at high temperature with a long lifetime which is highly desirable for in-situ long-term monitoring applications. The ability to form patterned planar coatings also facilitates the production of waveguide sensors which use combinations of clad materials of different specificities to detect a chemical species of interest.